The invention relates to improvements in fluid flow machines in general, and more particularly to improvements in so-called canned motor pumps wherein a can (normally a tube made of high-quality steel or the like) is installed between the driving and driven units of the motor which rotates one or more pumping elements. The purpose of the can is to hermetically seal the driving and driven units from each other.
German Utility Model No. 82 08 046 discloses a canned motor pump wherein the can comprises a single tubular wall which is mounted in the pump housing to seal the driving unit from the driven unit. The can is designed with a view to offer a minimum of resistance to magnetic torque transmitting forces between the driving and driven units of the pump motor. Such pumps are frequently used to convey highly aggressive or otherwise harmful or dangerous fluid media which must be hermetically sealed from the surrounding area in order to avoid damage to or destruction of the pump and/or injury to the attendants.
Presently known canned motor pumps of the type disclosed in German Utility Model No. 82 08 046 exhibit a number of serious drawbacks. For example, such pumps are likely to permit leakage of conveyed fluid media into the surrounding area in the event of leakage of the can. Such leakage can develop for any one of a number of different reasons including damage to the bearings for the driven unit, corrosion of the material of the can as a result of contact with chemically aggressive fluid media and/or contamination of the conveyed fluid medium, for example, by solid particles which rub against the can and cause the development of leaks and/or improper centering of the driving and driven units relative to each other and/or relative to the can. If the means for transmitting torque between the driving and driven units of a canned motor pump is a magnetic coupling, the support for the outer magnet or magnets must be mounted with a very high degree of accuracy; even small deviations of the position of such support from an optimum position can entail extensive damage to the can and leakage of conveyed fluid medium into the surrounding area. The situation is aggravated because the can is normally extremely thin in order to reduce magnetic losses between the driving and the driven units of the pump motor. Any (even very slight) leakage of certain types of conveyed fluid media can result in substantial damage to or in total destruction of the pump, injury to the attendants, ecological damage and/or damage to external components of the pump as well as to components which are connected to or adjacent the pump.
The aforementioned German Utility Model discloses specially designed seals which are to prevent the escape of conveyed fluid media in the event of damage to the can. This publication further discloses means for turning off the pump in the event of damage to the can. However, such undertakings are not entirely satisfactory and sufficient under all circumstances of use, for example, because the seals which are proposed in the Utility Model are normally dynamic seals which offer only limited protection against escape of conveyed fluid medium.